JP4996686B2 - Removal of carbon dioxide from flue gas - Google Patents

Description

  The present invention relates to an absorbent and a method for removing carbon dioxide from a gas stream, in particular from flue gas or flue gas.

  The removal of carbon dioxide from flue gas is desired for a variety of reasons, in particular, but in order to reduce carbon dioxide emissions, which are considered the main cause of the so-called greenhouse effect.

  On an industrial scale, aqueous solutions of organic bases such as alkanolamines are frequently used as absorbents to remove acidic gases such as carbon dioxide from gas streams. In that case, upon dissolution of the acidic gas, an ion product is formed from the base and the acidic gas component. The absorbent can be regenerated by heating, releasing to a lower pressure or stripping, where the ionic product reacts back to acid gas and / or the acid gas is stripped off by steam. After the regeneration step, the absorbent can be reused.

The flue gas has a very low carbon dioxide partial pressure. This is because they generally occur at pressures near atmospheric pressure and typically contain only 3-13% by volume of carbon dioxide. To achieve effective removal of carbon dioxide, the absorbent must have a high CO ₂ load capacity at low partial pressures. On the other hand, it is desirable that the absorption of carbon dioxide is not too exothermic in the course: the load capacity of the absorbent decreases with increasing temperature, so that the temperature rise caused by the high absorption reaction enthalpy is Is a drawback. In addition, energy consumption is increased during regeneration of the absorbent due to the high absorption reaction enthalpy. For obvious reasons, it is desirable that the amount of energy required to regenerate the absorbent (eg expressed as 1 kg of steam per kg of CO ₂ removed) is as low as possible.

  During cleaning of flue gas, typically large gas volumes are processed at low pressure, so it is desirable that the absorbent further have a low vapor pressure to keep the amount of absorbent loss low. In addition, it is desirable that the absorbent does not exhibit undesired interactions with other typical constituents of the flue gas, such as nitrogen or oxygen.

  Often, an aqueous solution of monoethanolamine (2-aminoethanol) is used for cleaning the flue gas. Monoethanolamine is inexpensive and has a high load capacity due to carbon dioxide. However, the amount of absorbent loss is high. Because monoethanolamine has a relatively high vapor pressure and shows a tendency to decompose at elevated temperatures in the presence of oxygen, the required amount is 1. 6-2.5 kg. The energy required for regeneration is high.

  Under the name Alkazid M, absorbents based on N-methylalanine potassium salt (potassium-α-methylaminopropionate) are known. It can be loaded as high as monomethanolamine. Amino acid salts have negligible vapor pressure based on their ionic structure. The disadvantage is that the energy required for regeneration is high, similar to monoethanolamine.

Although secondary and tertiary amines such as diethanolamine, diisopropanolamine or methyldiethanolamine cannot be loaded so high at low CO ₂ partial pressures, and therefore sometimes higher circulation rates are required. Instead, they can be regenerated with little energy consumption (1 kg of steam per kg of carbon dioxide removed). The disadvantage is their poor stability in the presence of oxygen.

EP-A 671200 describes the removal of CO ₂ from combustion gases at atmospheric pressure using an aqueous solution of an amino acid metal salt and piperazine. The amino acid metal salts described are potassium-dimethylaminoacetate and potassium-α-methylaminopropionate.

  Combustion gases usually contain trace amounts of nitrogen oxides or glass. They can easily form stable nitrosamines with secondary amines such as piperazine. Nitrosamine is a generic name for secondary amine N-nitroso compounds. They belong to extremely strong carcinogenic (cancer-causing) substances. The cancer-causing action is based on reactive metabolites of nitrosamines in substance alternation that can react with DNA of genetic material, thereby damaging the DNA and causing tumors. Therefore, as long as technically avoidable, efforts are made to prevent nitrosamines from being taken into the environment.

The problems underlying the present invention are absorbents, and (i) reduced potential for the formation of harmful nitrosamines, (ii) high CO ₂ absorption rate, (iii) high CO ₂ absorption capacity, (iv) Demonstrating a method for the removal of carbon dioxide from a gas stream, in particular flue gas, characterized by low energy requirements for regeneration, (v) low vapor pressure and (vi) stability in the presence of oxygen. is there.

［式中、Ｒ^１およびＲ^２は、互いに無関係に、アルキルまたはヒドロキシアルキルであり、
Ｒは、水素、アルキルまたはヒドロキシアルキルであり、または基ＲはＲ^１とともにアルキレンを形成し、
Ｍは、アルカリ金属であり、かつ
ｎは、１〜６の整数である］の少なくとも１つのアミノ酸塩、および
（Ｂ）少なくとも１つの第一級アルカノールアミン
の水溶液を包含する吸収剤に関し、その際、該吸収剤は、無機塩基性塩を本質的に含まない。 The present invention
(A) Formula (I)

Wherein R ¹ and R ² are, independently of one another, alkyl or hydroxyalkyl;
R is hydrogen, alkyl or hydroxyalkyl, or the group R together with R ¹ forms alkylene,
M is an alkali metal and n is an integer from 1 to 6], and (B) an absorbent comprising an aqueous solution of at least one primary alkanolamine, The absorbent is essentially free of inorganic basic salts.

R ¹ and R ² are generally C ₁ -C ₆ -alkyl or C ₂ -C ₆ -hydroxyalkyl, preferably methyl or ethyl. R is hydrogen, alkyl (eg C ₁ -C ₆ -alkyl) or hydroxyalkyl (eg C ₁ -C ₆ -hydroxyalkyl). n is an integer of 1 to 6, preferably 1 or 2. The group R together with R ¹ may be alkylene (eg C ₂ -C ₄ -alkylene).

  In addition, the present invention relates to a method for removing carbon dioxide from a gas stream, wherein the gas stream is contacted with an absorbent as defined above. In one advantageous embodiment, the partial pressure of carbon dioxide in the gas stream is less than 500 mbar, for example 50 to 200 mbar. The gas stream may contain oxygen (usually 0.5-6% by volume) and trace amounts of nitrogen oxides.

  The following embodiments relating to the process according to the invention apply accordingly to the absorbent according to the invention and vice versa, if not otherwise readable from the context.

The amino acid salts used according to the invention have a tertiary amino group. It is characterized by less heat of absorption compared to amino acid salts with primary or secondary amino functions. The heat of absorption of potassium dimethylamino acetate is, for example, about 17% lower than the heat of absorption of potassium α-methylaminopropionate. The lower the heat of absorption, the less the temperature rise in the absorber. In addition, the regenerative energy per kg of CO ₂ removed is less.

Suitable amino acid salts are, for example, α-amino acids such as N, N-dimethylglycine (dimethylaminoacetic acid), N, N-diethylglycine (diethylaminoacetic acid), N, N-dimethylalanine (α-dimethylaminopropionic acid), N, N-dimethylleucine (2-dimethylamino-4-methylpentane-1-acid), N, N-dimethylisoleucine (α-dimethylamino-β-methylvaleric acid), N, N-dimethylvaline (2- Dimethylamino-3-methylbutanoic acid), N-methylpyrroline (N-methylpyrrolidine-2-carboxylic acid), N, N-dimethylserine (2-dimethylamino-3-hydroxypropane-1-acid),
β-amino acids such as 3-dimethylaminopropionic acid, N-methyliminodipropionic acid, N-methylpiperidine-3-carboxylic acid,
Or, it is an alkali metal salt of an aminocarboxylic acid such as N-methylpiperidine-4-carboxylic acid or 4-dimethylaminobutyric acid.

  If the amino acid has one or more chiral carbon atoms, the structure is not taken into account; not only pure enantiomers / diastereomers, but any mixtures or racemates may be used.

  The alkali metal salt is preferably a sodium salt or a potassium salt, of which the potassium salt is most advantageous.

Particularly advantageous amino acid salts (A) are:
N, N-dimethylaminoacetic acid-potassium salt,
N, N-diethylaminoacetic acid-potassium salt and N-ethyl-N-methylaminoacetic acid-potassium salt.

As component (B), the absorbent according to the invention contains a primary alkanolamine. The primary alkanolamine acts as an activator and promotes CO ₂ uptake of the absorbent by intermediate carbamate formation. Compared to secondary amines, primary alkanolamines, for example, do not form undesired nitrosamines with nitrogen oxides that may be present in the gas stream to be treated.

  The alkanolamine (B) has at least one primary amino group and at least one hydroxyalkyl group. It typically contains 2 to 12 carbon atoms, preferably 2 to 6 carbon atoms. One or more oxygen atoms may be contained in the ether bond.

The alkanolamine (B) is preferably
2-aminoethanol,
3-aminopropanol,
4-aminobutanol,
2-aminobutanol,
5-aminopentanol,
2-aminopentanol,
Selected from 2- (2-aminoethoxy) ethanol.

  Among them, 4-aminobutanol, 2-aminobutanol, 5-aminopentanol and 2-aminopentanol are particularly advantageous due to their low vapor pressure.

In general, the absorbent is
Amino acid salt (A) 15-50% by weight, preferably 20-40% by weight, in particular 30-40% by weight and alkanolamine (B) 2-20% by weight, preferably 5-15% by weight, in particular 5-5% Contains 10% by weight.

  The absorbent may also contain additives such as corrosion inhibitors, enzymes and the like. Generally, the amount of such additives is in the range of about 0.01 to 3% by weight of the absorbent.

  The absorbent of the aqueous solution is essentially free of inorganic basic salts, ie it generally contains less than about 10% by weight, preferably less than about 5% by weight and in particular less than about 2% by weight. Contains. Inorganic basic salts are, for example, alkali metal carbonates or alkaline earth metal carbonates or alkali metal hydrogen carbonates or alkaline earth metal hydrogen carbonates, in particular potassium carbonate (potassium). Of course, metal salts of aminocarboxylic acids can be obtained by in situ neutralization of aminocarboxylic acids and inorganic bases, such as potassium hydroxide; however, this is essentially due to neutralization. No amount of base is used in excess of that required for.

A gas stream is generally a gas stream formed in the following manner:
a) Oxidation of organic substances such as flue gas or flue gas,
b) composting and storage of waste containing organic substances, or c) decomposition of organic substances by bacteria.

  Oxidation may be carried out under flame generation, ie as conventional combustion or as flame-free oxidation, for example in the form of catalytic oxidation or partial oxidation. The organic material subjected to combustion is usually a fossil fuel, such as coal, natural gas, petroleum, gasoline, diesel oil, raffinate or kerosene, biodiesel or waste with components of organic material. The starting material for catalytic (partial) oxidation is, for example, methanol or methane, which can be converted to formic acid or formaldehyde.

  Waste that is oxidized, composted or stored is typically household waste, plastic waste or package waste.

  The combustion of organic materials is most often performed using air in a normal combustion plant. Composting and storage of waste containing organic materials is generally performed in a garbage dump. Advantageously, the exhaust or exhaust of such a plant can be treated according to the method according to the invention.

  As organic substances for decomposition by bacteria, manure, straw, organic fertilizer, sludge, fermentation residue, etc. are usually used. Degradation by bacteria is performed, for example, in a normal biogas plant. Advantageously, the exhaust of such a plant can be treated according to the method according to the invention.

  The method is also suitable for treating exhaust gas from chemical synthesis plants used for (partial) oxidation of fuel cells or organic substances.

  Moreover, the process according to the invention naturally takes place during the mining of unburned fossil fuel gas, for example natural gas, for example so-called coal bed gas, ie coal; for treating the gas to be collected and compressed It can also be applied.

In general, these gas streams contain less than 50 mg / m ^{3 of} sulfur dioxide at standard conditions.

  The starting gas may have either a pressure approximately corresponding to the pressure of the ambient air, ie a standard pressure, or a pressure showing a deviation from the standard pressure up to 1 bar.

  Suitable equipment for carrying out the process according to the invention is at least one washing tower, such as irregular packed towers, ordered packed towers and plate towers, and / or other absorbers, such as membrane contact devices, radial stream scrubbers , Jet scrubbers, venturi scrubbers and rotary spray scrubbers. In this case, the treatment of the gas stream with the absorbent is preferably carried out in countercurrent in the washing tower. In that case, the gas stream is generally supplied to the lower region of the tower and the absorbent is supplied to the upper region of the tower.

  Suitable for carrying out the process according to the invention is also a washing tower made of plastic, for example polyolefin or polytetrafluoroethylene, or a washing tower whose internal surface is entirely or partly coated with plastic or rubber. It is. Furthermore, a membrane contact device with a plastic casing is suitable.

  The temperature of the absorbent is generally about 25 ° C. to 70 ° C. in the absorption process, and when a column is used, for example, 25 ° C. to 60 ° C., preferably 30 ° C. to 50 ° C. and especially advantageous when the column is used Is 35 ° C. to 45 ° C. and, for example, 40 ° C. to 70 ° C. at the bottom of the column. A product gas is obtained which is poor in carbon dioxide and other acid gas components, i.e. in which these components are depleted, and an absorbent loaded with acid gas components.

From absorbent gas component of acid-loaded, resulting carbon dioxide is released in the regeneration step, in which regenerated absorbent is obtained. In the regeneration process, the load on the absorbent is reduced and the resulting regenerated absorbent is preferably subsequently returned to the absorption process.

In general, the loaded absorbent is
a) heating to eg 70-110 ° C.
b) pressure release,
c) Regenerated by stripping with an inert fluid or a combination of two or all of these measures.

  In general, the loaded absorbent is heated for regeneration and the released carbon dioxide is separated off, for example in a desorption tower. Before the regenerated absorbent is again introduced into the absorber, it is cooled to a suitable absorption temperature. In order to utilize the energy contained in the absorbent regenerated at high temperature, it is advantageous to preheat the loaded absorbent from the absorber by heat exchange with the high temperature regenerated absorbent. By heat exchange, the loaded absorbent is brought to a higher temperature, so that less energy is used in the regeneration process. By heat exchange, partial regeneration of the loaded absorbent can already be carried out under the release of carbon dioxide. The resulting gas-liquid mixed phase stream is conducted into a phase separation vessel and carbon dioxide is removed from the phase separation vessel; the liquid phase is conducted into a desorption tower for complete regeneration of the absorbent. .

  Often, the carbon dioxide released in the desorption tower is subsequently compressed and fed, for example, to a pressure tank or a sequestering section. In these cases it may be advantageous to carry out the regeneration of the absorbent at an elevated pressure, for example 2 to 10 bar, preferably 2.5 to 5 bar. For this purpose, the loaded absorbent is compressed to the regeneration pressure by a pump and introduced into the desorption tower. Carbon dioxide is thus produced at higher pressure levels. The pressure difference with respect to the pressure level in the pressure tank is smaller and in some cases the compression stage can be reduced. Higher pressures during regeneration require higher regeneration temperatures. For higher regeneration temperatures, a lower residual load of the absorbent can be achieved. The regeneration temperature is generally limited only by the thermal stability of the absorbent.

  Prior to the absorbent treatment according to the invention, the flue gas is preferably subjected to washing with an aqueous liquid, in particular with water, in order to cool and wet (quenching) the flue gas.

  During cleaning, dust or gaseous impurities such as sulfur dioxide can also be removed.

Claims (10)

(A) Formula (I)

Wherein R ¹ and R ² are, independently of one another, alkyl or hydroxyalkyl, and R is hydrogen, alkyl or hydroxyalkyl ; or the group R together with R ¹ forms an alkylene and R ² Is alkyl or hydroxyalkyl;
M is an alkali metal and n is an integer from 1 to 6] at least one amino acid salt of 15-50% by weight, and (B) at least one primary alkanolamine 2-20% by weight
The carbon dioxide absorbent includes an aqueous solution of an alkali metal carbonate, an alkaline earth metal carbonate , an alkali metal bicarbonate or an alkaline earth metal bicarbonate. Carbon dioxide absorbent containing less than mass%.   The carbon dioxide absorbent according to claim 1, wherein the carbon dioxide absorbent contains 20 to 40% by mass of an amino acid salt (A) and 5 to 15% by mass of an alkanolamine (B).   The carbon dioxide absorbent according to claim 1 or 2, wherein M is potassium.   The carbon dioxide absorbent according to any one of claims 1 to 3, wherein n is 1 or 2. Amino acid salt (A)
N, N-dimethylaminoacetic acid-potassium salt,
The carbon dioxide absorbent according to any one of claims 1 to 4, which is selected from N, N-diethylaminoacetic acid-potassium salt and N-ethyl-N-methylaminoacetic acid-potassium salt. Alkanolamine (B)
2-aminoethanol,
3-aminopropanol,
4-aminobutanol,
2-aminobutanol,
5-aminopentanol,
2-aminopentanol,
The carbon dioxide absorbent according to any one of claims 1 to 5, which is selected from 2- (2-aminoethoxy) ethanol.   A method for removing carbon dioxide from a gas stream, wherein the gas stream is contacted with an absorbent according to any one of claims 1 to 6 and the absorbent is loaded with carbon dioxide. How to remove carbon dioxide from a stream.   8. A process according to claim 7, wherein the partial pressure of carbon dioxide in the gas stream is less than 500 mbar.   9. A method according to claim 7 or 8, wherein the gas stream contains oxygen. The absorbent loaded with carbon dioxide,
a) heating,
b) pressure release,
10. A method according to any one of claims 7 to 9, wherein the regeneration is by c) stripping with an inert fluid or a combination of two or all of these measures.